Recently, in a flat panel display device such as mobile phone or personal digital assistant (PDA), a thin sheet-like cover glass is disposed on a front surface of a display to extend in a wider region than the image display area with an aim to enhance the protection and beauty of the display.
Such the flat panel display device is required to be lightweight and thin and in order to meet this requirement, the thickness of a cover glass used for display protection is also required to be reduced.
However, decreasing the thickness of the cover glass causes the problems that the strength is reduced and the cover glass itself may be broken by dropping, etc. during use or carrying and therefore its primary role of protecting the display device cannot be fulfilled.
Accordingly, in the conventional cover glass, with an aim to improve the scratch resistance, a compressive stress layer is formed on the surface by chemically strengthening a float glass produced by a float method, and the scratch resistance of the cover glass is thereby enhanced.
It has been reported that warpage occurs in a float glass after chemical strengthening and impairs the flatness (Patent Document 1). The warpage is thought to occur due to the difference in the chemical strengthening behavior between a glass surface (hereinafter, also referred to as top surface) that is out of contact with molten tin at the time of float forming, and a glass surface (hereinafter, also referred to as bottom surface) that comes into contact with molten tin.
Heretofore, the reason for the difference in the chemical strengthening behaviors between the top surface of the float glass and the bottom surface has been considered to be the invasion of a molten metal into the glass surface in contact with the molten metal at the time of float forming (Patent Document 1).
In Patent Document 1, it is disclosed that the warpage is improved when a sheet-like material produced by a float method and processed is, without applying surface polishing, dipped in or put into contact with Li ion, Na ion or a mixed inorganic salt thereof and then chemically strengthened.
Furthermore, in order to reduce the warpage, conventionally employed is a coping method of decreasing the strengthening stress produced by chemical strengthening or performing chemical strengthening after subjecting the top surface and the bottom surface of a float glass to a grinding treatment, a polishing treatment, etc. so as to remove a surface heterogeneous layer.